RECENT RESEARCH ON INDIGO 577 



never be known with certainty, and consequently afforded 

 unsatisfactory guidance as to the best courses to adopt and 

 the direction which improvements should take. It was to the 

 working out of such analytical methods that Mr. Bloxam and 

 his assistants devoted their attention, with the results described 

 in the present Report. 



The problem of finding out the percentage of a plant product 

 in a mixture is not easy, and in the case of indigotin is made 

 harder by the fact that this substance is only sparingly soluble 

 in liquids other than those of high boiling-point ; thus its 

 separation from other substances is not easily accomplished, as 

 these go more or less into solution in the solvent used. Mr. 

 Bloxam's solution of the problem is to heat the crude indigo 

 with fuming sulphuric acid containing a certain proportion of 

 sulphuric anhydride ; this converts the indigotin into indigotin- 

 tetrasulphonic acid, which forms a blue solution when the liquid 

 is diluted with water. This acid, it was found, has the property 

 of forming a potassium salt which is practically insoluble in 

 cold strong potassium acetate solution ; accordingly on mixing 

 the blue solution of the acid with strong solution of potassium 

 acetate the potassium salt is formed by double decomposition, 

 and when formed is precipitated owing to its insolubility in the 

 excess of potassium acetate solution. The precipitate can be 

 got into solution by the aid of heat, and on cooling with ice 

 separates again in a crystalline form which can be easily 

 filtered off and washed with cooled potassium acetate solution ; 

 the filtrate and washings contain only a trace of indigotin. 

 The precipitate on the filter is then dissolved in hot water, and 

 the amount of indigo in it is determined volumetrically by using 

 potassium permanganate or titanium trichloride. The solutions 

 are standardised by means of indigotin obtained in a pure state 

 by sublimation ; and precautions to be taken in performing this 

 last operation are given by the authors. The merit of the method 

 lies in the fact that the precipitation and washing of the indigotin 

 compound afford an opportunity of getting rid of impurities 

 which would otherwise render the volumetric results incorrect. 



In order to make sure of the effectiveness of the method in 

 separating impurities the authors used preparations of various 

 impurities occurring in indigo, namely indigo-gluten, indigo- 

 brown, and the indigo-yellow or kaempferol, which latter 

 substance occurs in the indigo from the Natal or Java indigo 



